Various types of circuits are provided with decoupling capacitors which provide local energy storage and decouple one part of a circuit from another to reduce noise. Examples of such circuits include (but are not limited to) power management components such as amplifiers, drivers, charge pumps, low dropout regulators (LDOs), buck converters and boost converters. It is important to limit the inrush current for these types of circuits that occurs when a power supply is first switched on. At that point, capacitors or other storage elements forming part of the circuit are not charged up and so the high peak currents that result can damage the circuitry, or cause operation of the circuit to be unreliable or to fail altogether.
Circuits of this type can for example be found as component parts of a power management integrated circuit (PMIC) which is provided to manage the power requirements of a host system and which may comprise various sub-blocks such as low drop-out voltage regulators (LDO), DC-DC buck convertors, DC-DC boost converters and so on.
An example use-case for a PMIC is shown in FIG. 1. Here, a mains charger circuit 100 powers a PMIC 102 and a battery 104. A switch 106 can be used to charge the battery 104 when the charger 100 is attached or be used in absence of charger 100 to power the PMIC 102 from the battery 104.
Consider a scenario where the charger circuit 100 is charging both the battery 104 and powering the PMIC 102. The maximum current from the charger is I1. Under no condition should I2+I3 become more than I1. If that happens, the charger circuit 100 will be overloaded and the output voltage from the charger will fall causing the PMIC to reboot.
The PMIC comprises various sub-components which in this illustration comprise one or more LDOs 108 and one or more buck convertors 110. The PMIC sub-components comprise output decoupling capacitors which need to be charged when the sub-component is enabled. The maximum current at their start-up would be limited only by the maximum current of the sub-component (buck or LDO) circuit. If this current is more than I1−I3, which it could be, the system may shutdown and go into a loop of starting and shutting down.
To avoid a situation like this the start-up current for the sub-blocks of PMIC need to be regulated. It is desirable for this regulation to be independent of one or more of supply, process and temperature.
Charger circuit 100 and battery 104 each have an output impedance, bandwidth and maximum current capability. As these components are external to PMIC these parameters may vary considerably. When any of the sub-blocks in PMIC are enabled during the battery charging process, the current at start-up would come from supply decoupling capacitors at the input of PMIC. This would require a large value decoupling capacitor which would occupy significant area on a printed circuit board (PCB). This would be very expensive, in particular for a handheld electronic device where there is great pressure to minimise PCB area occupied by each circuit.
If the sub-block start-up current could be well regulated, and the time taken to reach the maximum regulated current at start-up be controlled, this would additionally allow a reduction in the size of the supply decoupling capacitor too.
Further, a very sharp edge in the start-up current can generate undesired effects in other components that are supplied by the PMIC. For example, in audio applications this effect may generate harmonics in the audio frequency and may cause interference with the audio. This effect is independent of the charging status.